1. Field of the Invention
The present invention relates to a creasing apparatus and an image forming system and, more specifically, to a creasing apparatus that forms a crease on a sheet-like member (hereafter, referred to as a “sheet”) that has been conveyed from upstream before the sheets in a bundle are bound together at the central portion thereof and folded in two about the central portion and relates to an image forming system that includes the creasing apparatus and an image forming apparatus, such as a copying machine, printer, facsimile, or digital multifunction peripheral that has the functions of the above apparatuses in combination.
2. Description of the Related Art
Conventionally, a bundle of sheets is obtained by combining sheets that are discharged from an image forming apparatus, the sheets in the bundle are then bound together at the central portion thereof, and the bundle of center-bound sheets is folded in two at the central portion, i.e., what is called center-folding or center-folded bookbinding is performed. If sheets in a bundle are folded as one, the folded portion of the outer sheet of the bundle is stretched to a larger extent than that of the inner sheet. Because the image-formed area on the folded portion of the outer sheet is stretched, damage, such as toner coming off, may occur on the image area. A similar phenomenon occurs in other folding processes such as Z-folding, tri-folding, or the like. A sheet in a bundle may be insufficiently folded due to the thickness of the bundle.
A creasing apparatus, called a creaser, to form a crease on the folding portion of the sheet in advance is already known. Before a folding process, such as a double folding, on a bundle of sheets is performed, a crease can be formed even on the outer sheet by using a creaser, so that the outer sheet can be easily folded to prevent toner from coming off the outer sheet. In such a creasing apparatus, a crease is formed on a sheet in a direction perpendicular to a sheet conveying direction by using a method, such as driving a roller against a sheet, heating a sheet with a laser, or pressing a sheet with a creasing blade.
For example, in order to form a crease with a good shape and with high accuracy in accordance with the type of sheet, Japanese Patent Application Laid-open No. 2008-81258 discloses a configuration that allows a roller for forming a crease to be replaceable with an optimal roller in accordance with the sheet. Furthermore, a perforation method is also known already to perform a perforating process on a sheet after an image is formed on the sheet to allow an easy cutting of the sheet at the position of the perforation.
If a single printing system uses an apparatus that performs a creasing process and an apparatus that performs a perforating process, the two apparatuses need to be arranged along the sheet conveying direction; therefore, the printing system needs to have a space for installing each apparatus to result in a larger installation space for an entire system.
If a folding process is performed on an area of a sheet on which a perforating process has been performed, a gap between the perforated area and the folded area often occurs because the conveyed sheet is stopped at each position of a process. Because it is necessary to convey a sheet so that the two areas coincide with each other, a conveying speed is reduced and a processing efficiency is decreased accordingly.